RHEEM gas furnace pressure switch wont work

My RHEEM horizontal gas furnace will not heat. When thermostat calls for heat, the exhaust blower comes on but nothing else happens. I had service tech to check it out. He said I needed a new pressure switch, the one that sends signal to the igniter. I replaced pressure switch (P1 on schematic) with a new one, but get the same results. There is not enough vacuum from the exhaust blower to activate the pressure switch. I have disconnected the flue from blower so I know flue obstruction is not the problem. I can manually operate the pressure switch and furnace operates as it should. Any thoughts on how to increase vacuum?

Problem: Furnace ignitor will not glow. If a furnace has a bad ignitor, open limit switch, open rollout switch, open pressure switch or bad control board. This is what I see in the furnace sequence of operation:

1. Thermostat calls for heat. You should be getting 24 to 28 volts AC between the W (white) and C (com) terminals on your control board. *Make sure furnace safety switch is pressed in when testing.

2. Draft inducer motor starts. If draft inducer does not start then you either have a thermostat problem, thermostat wiring problem, transformer problem, bad relay on control board or a draft inducer problem.

3. Pressure switch attached by a small plastic or rubber tube to the draft inducer senses the negative pressure produced by the draft inducer and closes. Should have 24 volts to ground across both terminals on the pressure switch. If pressure switch is not closed with draft inducer running check for stopped up vent or stopped up drain line on a condensing furnace.

4. Limit Switch and rollout switch/switches should all be closed. Press in on the reset button in the center of each rollout switch to make sure each rollout has been reset. You should have 24 to 28 volts from each terminal to ground. On most furnaces the ignitor will not glow unless all the safety controls (limit, rollout switch/switches, pressure switch) are closed. If a limit or rollout switch is open then you might have an over heating problem and a possible dangerous condition. If the Limit or rollout continues to trip I would strongly recommend calling an HVAC service company to check your furnace out.

5. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. If you hear a hissing gas sound then more than likely either the ignitor is broken or the relay on the control board that sends power to the ignitor is broken. If the ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace will shut down, and try ignition again. Most of the time after three tries the furnace will and go into a lock out condition until you turn your power switch back off and on again.

Solution: You might need a new ignitor, rollout switch, limit switch, pressure switch or control board